The front foot portion typically has a small thickness. On the other hand, the front foot portion is bent significant and repeatedly at the MP joint, or the like. In areas where this bending is repeated, the mid sole eventually undergoes permanent deformation. Particularly, the permanent deformation is likely to occur in the upper layer of the front foot portion.
The middle foot portion supports the arch of the foot. The arch has significant individual variations. Wearers having low arch are likely to feel an upthrust against the arch, whereas wearers having high arch may have their arch drop.
When a shoe lands on the ground, a largest impact load acts upon the foot sole via the sole therebetween on the lateral side of the rear foot portion. This is referred to as the 1st strike, and it is important to absorb the impact of the 1st strike.
A mid sole of a layered structure is likely to exert other functions as compared with a mid sole of a single-layer structure.